Dietary phytochemicals are potential sources of new drug leads to prevent breast and other cancers. Nuclear Receptor Alternative Site Modulators (NRAMs) are a recently defined class of chemicals with potential application in preventing breast cancer by blocking estrogen receptor (ER) function through mechanisms other than blocking ligand activation. NRAMs are distinct from currently used chemopreventives such as selective ER modulators (SERMs, e.g., tamoxifen (TAM)) and aromatase inhibitors (AI, e.g. letrozole) against which tumor cells become resistant. We recently reported that a purified anacardic acid (AnAc 24:1n-5) inhibited the proliferation of ER-positive/TAM-responsive MCF-7 and ER-positive/endocrine-resistant LCC9 and LY2 breast cancer cells, but not primary normal human mammary epithelial cells (HMECs). AnAc 24:1n-5 does not compete with [3H]estradiol (E2) for ER or ER, but inhibits ER-DNA interaction and transcriptional activity with greater efficacy than ER. These data indicate that AnAc 24:1n-5 functions as a NRAM. AnAc 24:1n-5 is a congener of AnAc constituents found in caju juice (commonly consumed in Brazil) and thus establishes dietary intake as a relevant route for this potential NRAM. Our breast cancer cell-based work provides strong rationale for the proposed pilot study that addresses PAR-11-079 by 1) Investigating the mechanisms of action of AnAc as an NRAM by utilizing ER overexpression and shRNA knockdown approaches to confirm the specificity of AnAc 24:1n-5 as an NRAM. 2) Pilot testing and developing AnAc as a dietary intervention to prevent or retard carcinogen (NMU)-induced mammary tumors in female Sprague- Dawley (SD) rats. 3) Developing sensitive and reliable methods for analysis of AnAc and its metabolites in serum, tumors, and tissues. 4) Determining the bioavailability and dose-response of AnAc as foods, nutrients, and other naturally-occurring food constituents. The hypotheses tested in these Specific Aims is that 1) AnAc acts as an ER-selective NRAM and 2) that AnAc, supplied orally in AIN-93M diet will function as a natural chemopreventive to NMU-induced primary mammary tumors in female SD rats. These studies will provide the first in vivo data regarding the efficacy of AnAc in primary mammary tumor prevention and will establish the serum and tissue levels of AnAc after oral consumption.